Conventionally, casings and flanges of a machinery such as steam turbine or the like are mutually joined by means of junction or fastening bolts, and the clamping force of the junction bolts are preliminarily adjusted so as to maintain a predetermined clamping force in consideration of a temperature at the operation time of the machinery.
An example in which the conventional junction bolts of the type described above are utilized for the joined portion of casings of a steam turbine will be described hereinbelow for the better understanding of this technical art of field.
A steam turbine is generally equipped with a pair of upper and lower casing parts of inner and outer casings which are mutually joined by the junction bolts, and a force acting on the junction bolt is influenced with not only steam pressures in the inner and outer casings but also tensioning force in the axial direction of the junction bolt for strongly fastening the upper and lower casing parts. In addition, a stress resisting against thermal deformation due to high termperature steam at a turbine operation time will also be considered. These forces and stresses acting on the junction bolts will constitute considerably large values.
In order to support or hold such a large stress, the size or diameter of the junction bolt is made large. For example, with a power generation steam turbine having power generation capability of about 500 MW, junction bolts each having a diameter of 165 mm are used for joining the casing parts of the inner and outer casings. It is therefore difficult to clamp these large junction bolts in use of an ordinary clamping tool.
In a prior art technique, taking the above defect into consideration, is adopted a method in which an electric heater is inserted into a hollow hole bored in the junction bolt thereby to heat the junction bolt and hence to temporarily extend the same in the axial direction thereof. The thus extended junction bolt is then clamped by means of clamping nuts. This method, however, requires much time and labour for inserting and drawing the heater into and out of the hollow hole of the junction bolt.
In another method of the conventional technique, highly heated air or specific gas of high heat capacity is fed into the hollow hole of the junction bolt in an alternation of the electric heater to thereby axially extend the junction bolt. This method, however, also involves a problem of requiring much time to heat the junction bolt to the desired temperature. With a technical view point, this method is in principle based on the difference of thermal expansion between the flange portions of the casings of the steam turbine and the junction bolts for joining the same. Accordingly, a necessary procedure will have to be taken for preventing the flange portions from temperature rising. In actual operation, for this purpose, the number of the junction bolts to be simultaneously heated must be limited to locally carry out the bolt heating operation, which adversely results in extended and divided working times, thus being noneconomical and reducing the working efficiency.
Moreover, in the method in which a heated air or gas is utilized, the respective members or parts are likely to be subjected to the thermal deformation because of the high temperature heating of the junction bolts, and the substances of the members will be themselves degraded. Particularly, in the high temperature gas heating method, there is a high possibility of corrosion of the inner surface of the hollow hole of the junction bolt due to the high temperature oxidization based on the used gas, and there is a fear of lowering the strength of the junction bolt.
Conventionally, the method of thermally elongating the junction bolt and clamping the same in the extended state has been generally adopted in a case where a clamping torque of the extent that cannot be obtained by the manual force, for example, more than about 50 kg.times.m/s.sup.2, is required. Particularly, with the steam turbine, this method is generally adopted in a case where a junction bolt having a diameter of more than about 50 mm is utilized.
In another aspect of the prior technique for eliminating or simplifying the joining working or operation, although a torque wrench operated by a hydraulic mechanism has been utilized, a twist-back force will be applied to the junction bolt in use of the torque wrench. Accordingly, the junction bolt has to be designed in the strength, in consideration of the combined stress of the shearing stress due to the twist-back force and the tension stress due to the extension of the junction bolt. For this reason, it is required on the matter of design to restrict the allowable tension stress below about 83% of the possible durable force of the junction bolt, which is based on the generally known shearing stress energy theory, and which may result in a fear of ensuring no adequate clamping force for the junction bolt in a certain case. Moreover, the torque wrench equipped with the hydraulic mechanism constitutes a heavy equipment and requires a wide working space around the joining portion. In some case, it will be necessary to use a crane for moving or conveying the torque wrench, thus lacking in maneuverability of the working.
The problems or disadvantages described above in the prior technique are items to be improved in the assembling or disassembling working of the steam turbine mainly at a time of periodical inspection thereof. Additionally, other problems have been encountered in the prior technique at a time of actual operation of the steam turbine in a view point of the strength maintenance of the joined portions as described hereunder.
With the machinery such as steam turbines which is usually operated in a high temperature condition, junction means such as bolts has to be re-clamped optionally because the clamping force or clamped condition at the joined portions is varied as time passes due to the thermal expansion or creeping phenomenon.
As a matter of design, in general, it is considered to select the materials of the members to be joined and the junction bolts so that the thermal expansion efficiencies of the materials constituting both the members are approximately equal or similar to each other to counterbalance the mutual effect due to the thermal expansions of both the members.
In an actual operation, however, apart from the above-mentioned ideal condition, the clamping force of the junction bolt (i.e., the clamping stress to the junction bolt body) is often lowered as time passes by the synergism of the creeping phenomenon and the temperature variation in wide range. Particularly, in a case where the thermal expansion efficiency of the junction bolt is larger than that of the member to be joined, the clamping force of the junction bolt lowers as the operation temperature rises, which results in a fear of leaking a highly pressurised fluid such as steam. This tendency has been remarkably observed according to the development of the daily load-following turbine operation mode, for example, in which the operation temperature is frequently varied in a relatively short time interval in response to a required amount of the steam turbine.
In order to obviate the lowering of the clamping force of the junction bolt due to the thermal expansion, in an improved structure of a conventional steam turbine, a steam introduction tube is located to communicate respectively with a plurality of bolt holes formed in the flange portions of the casings so as to cool the junction bolt body or to heat the flange portions to be joined together by introducing steam of a predetermined temperature through the steam introduction tube, thereby preventing the lowering of the clamping force.
The flange portion, however, is generally formed with relatively large thickness and a wide surface area, so that it is considerably difficult to transfer the heat into the interior of the flange portion, and in a certain operating condition, the junction bolt body with a relatively small thermal capacity may be first extended before the flange portion is extended. This may result in the lowering of the clamping force, and in some adverse case, there is a fear of the leakage of the steam.